A) Field of the Invention
This invention relates to a crystal-growth substrate adapted for growing zinc-oxide-containing (ZnO-containing) compound semiconductor crystal, a ZnO-containing compound semiconductor device and a manufacturing method of the ZnO-containing compound semiconductor crystal.
B) Description of the Related Art
A ZnO-containing compound semiconductor such as ZnO is one of wide band gap semiconductors, and its exciton binding energy is large as about 60 meV. A device with higher emission efficiency than a light emitting diode using a gallium nitride-containing (GaN-containing) compound semiconductor as an active layer material can be theoretically obtained by a light emitting diode using the ZnO-containing compound semiconductor as the active layer material.
Therefore, the ZnO-containing compound semiconductor is expected as an active layer material of a blue light emitting diode or an ultraviolet light emitting diode. In order to use the ZnO-containing compound semiconductor as the active layer material of the light emitting diode, at first, it is necessary to obtain a single crystalline layer of the compound semiconductor.
The single crystalline layer of the compound semiconductor is formed, for example, by a molecular beam epitaxy (MBE) method or by a laser ablation deposition method, on an a-cut (a-plane) sapphire substrate or a c-cut (c-plane) sapphire substrate directly or via a template layer.
The crystal structure of the ZnO-containing compound semiconductor is a wurtzite-type that is one of the hexagonal crystal structures, and the crystal growth on an a-cut sapphire substrate or a c-cut sapphire substrate usually takes place in the direction of xe2x88x92c axis (oxygen (O ) surface). When, for example, a gallium (Ga) surface GaN film is used as a template layer, it is also possible to make the crystal growth of the ZnO-containing compound semiconductor carry out in the direction of +c axis (zinc (Zn) surface). Moreover, crystal growth can be carried out in the direction of +c axis on a Zn surface ZnO substrate.
In order to use ZnO-containing compound semiconductor as, for example, an active layer of a light emitting diode, it is desirable to obtain a single crystalline layer with less crystal defect. However, crystal defect tends to be produced in the single crystal of the ZnO-containing compound semiconductor.
It is an object of the present invention to provide a crystal-growth substrate adapted for a ZnO-containing compound semiconductor crystal by which a ZnO-containing compound semiconductor crystal with an improved crystallinity can be easily obtained.
It is another object of the present invention to provide a ZnO-containing compound semiconductor device with a ZnO-containing compound semiconductor single crystalline layer wherein the single crystalline layer with an improved crystallinity is easily obtained.
It is a further object of the present invention to provide a manufacturing method of the ZnO-containing compound semiconductor crystal by which a ZnO-containing compound semiconductor crystal with an improved crystallinity is easily obtained.
According to one aspect of the present invention, there is provided a growth substrate comprising a compound single crystal layer of a hexagonal crystal structure having a plurality of (0001) surfaces configured as a sequence of steps aligned along a direction of a-axis and adapted for growing, on said compound single crystal layer, a ZnO-containing compound semiconductor single crystal of a hexagonal crystal structure having a growing front surface declined toward the direction of the a-axis.
According to another aspect of the present invention, there is provided a ZnO-containing compound semiconductor device, comprising: a growth substrate comprising a compound single crystal layer of a hexagonal crystal structure having a plurality of (0001) surfaces aligned in a sequence of steps along a direction of a-axis; and a ZnO-containing compound semiconductor single crystal layer of a hexagonal crystal structure formed on said compound single crystal layer and having a growth front declined toward the direction of the a-axis.
According to further aspect of the present invention, there is provided a method of manufacturing a ZnO-containing compound semiconductor crystal, comprising the steps of: (a) preparing a crystal-growth substrate comprising a compound single crystal layer of a hexagonal crystal structure having a plurality of (0001) surfaces aligned in a sequence of steps along a direction of a-axis; and (b) growing a ZnO-containing compound semiconductor single crystal of a hexagonal crystal structure having a growing surface leaning toward the direction of the a-axis on said compound single crystal layer.
According to knowledge of the inventors of the present invention, it is desirable to grow up a ZnO-containing compound semiconductor in the direction of +c axis (Zn surface) in terms of improving crystallinity of the ZnO-containing compound semiconductor single crystal.
According to the above structure, it will become easy to obtain a ZnO-containing compound semiconductor crystal with improved crystallinity, and it becomes easy to improve performance of the semiconductor device using the ZnO-containing compound semiconductor single crystalline layer.